Perimetry is a very important and common test in clinical eye care, second only to the simple determination of visual acuity. It is used for examining the range and the sensitivity of a subject's visual field, e.g. in connection with diagnosis and treatment of glaucoma, for testing for neurological diseases, in mass screening etc.
In static threshold perimetry, the limit or threshold of a subject's light perception at a number of discrete locations in the visual field is determined. The test is conducted by means of a computerized perimeter, typically in the following way: the patient is seated in front of a perimeter and asked to look steadily at a centrally placed fixation target, e.g. on a screen or in a hemispheric bowl. Visual stimuli are presented successively with different intensities and at different locations. The patient is asked to press a response button every mime he perceives a stimulus, whether close to or distant from the fixation target, whether faint or strong.
There are different methods for presenting stimuli to the patient in order to establish his threshold for perception of light. One common method consists in showing a stimulus having an intensity close to the expected threshold value at the test location concerned. If the patient does not respond to the stimulus, the intensity of the subsequently presented stimuli is thereafter increased stepwise until a response is received from the patient, i.e. until a stimulus is seen. The first intensity level at which a response is received can be defined as the threshold of the test location concerned. The precision of the test can be increased by reversing the test process when the first response is received, and by continuing it in smaller steps until the first unseen stimulus is encountered. The threshold can then be defined as the average intensity level of the last seen stimulus and the first unseen stimulus. If, on the other hand, the patient responds to the first stimulus, the intensity is gradually decreased until no response is received, whereupon the test procedure is reversed. In this context, it should be pointed out that the test locations are usually tested in random order, a stimulus at a first test location being followed by a stimulus at another test location etc., and the next stimulus for the first test location being not presented until after several subsequent stimulus presentations.
When the threshold values for all the test locations in the visual field have been determined, they are often compared with normal, previously determined threshold values for patients of the same age to establish whether there are any deviations from normal, or with previous values for the same eye of the patient to establish whether a disease under observation has progressed or receded.
One problem inherent in static threshold perimetry is that it is time-consuming. As described above, several stimuli of different intensity are presented at each test location on the screen. Since usually 50-100 test locations are examined, one test consists of several hundred stimulus presentations. Typically, the time required for a complete static threshold perimetry test is about 10-20 minutes per eye. The test is very tiring, which may affect the test accuracy.
Furthermore, even though the test procedure is computerized, an operator must usually be present during the test to ensure that the patient maintains fixation, to encourage him and to answer any questions he may have.
Another problem of static threshold perimetry is the assessment of the responses from the patients. After each presentation of a stimulus, the perimeter waits for a response during at most a predetermined time period. If no response is received during the predetermined time period, the stimulus has usually not been seen, and if a response is received, the stimulus has usually been seen. However, it is well-known that patients sometimes press the button without having seen any stimulus at all, and they sometimes fail to press it despite having seen the stimulus. These kinds of responses are called false positive responses and false negative responses, respectively. It goes without saying that it is important to minimize the number of false positive responses and false negative responses to keep the measurement error level as low as possible. The frequency of false positive responses is examined by false positive catch trails. The perimeter acts as when displaying a stimulus without showing one and registers whether the patient responds or not. The frequency of false negative responses is examined by false negative catch trials. A strong, supraliminal stimulus is presented at a location where the threshold has already been measured and the perimeter registers whether the patient responds or not. Usually 20-30 catch trials are presented during a test.
Many suggestions have been made for shortening the total time required for performing static threshold perimetry. Most of them have aimed at reducing the number of presentations of stimuli. One example is given in Heijl A, Krakau CET: An automatic perimeter for glaucoma visual field screening and control; Albrecht v. Graefes Arch. Clin Exp. Ophthalmol 197:13-22, 1975. Another example is given in U.S. Pat. No. 4,334,738 describing a method for better selecting the intensity level for the first stimulus presented at each test location, whereby the number of presentations of stimuli required for arriving at a threshold value, can be reduced. A further example is disclosed in U.S. Pat. No. 4,927,259 where the intensity is varied by logarithmic steps between stimuli for one test location.
Fewer suggestions aim at shortening the time between the presentation of stimuli. Most of the testing time is in fact used for awaiting a response from the patient. Traditionally, a fixed maximum waiting time between stimuli is set before the commencement of the test, and is thereafter used throughout the test. If the patient does not respond to a stimulus, the perimeter must wait all the fixed maximum waiting time before the next stimulus is presented. If, on the other hand, the patient responds, a new stimulus can be presented a certain time after the response has been received. In general, it is not possible to shorten the fixed maximum waiting time between successively presented stimuli, for this would involve the risk of missing responses from slow patients, which would decrease the reliability of the test.
DE 31 35 384 discloses the use of a variable waiting time consisting of the patient's mean reaction time plus a statistically varying time addition. By this variable waiting time, it is taken into account that the patient's reaction time at the beginning of the test is shortened owing to test learning, and is prolonged at the end of the test owing to test tiredness. Also EP 0 163 674 describes the use of a variable waiting time based on the patient's mean reaction time. Here, the test is interrupted if the patient's reaction time increases too much, for, if so, the patient is considered to be tired and the test result to be unreliable.